Energy is often transmitted through or reflected from a medium to determine characteristics of the medium. For example, in the medical field, instead of extracting material from a patient's body for testing, light or sound energy may be caused to be incident on the patient's body and transmitted (or reflected) energy may be measured to determine information about the material through which the energy has passed. This type of non-invasive measurement is more comfortable for the patient and can be performed more quickly
Non-invasive physiological monitoring of bodily function is often required. For example, during surgery, blood pressure and the body's available supply of oxygen, or the blood oxygen saturation, are often monitored. Measurements such as these are often performed with non-invasive techniques where assessments are made by measuring the ratio of incident to transmitted (or reflected) light through a portion of the body, for example a digit such as a finger, or an earlobe, or a forehead.
Durable and disposable sensors are often used for such physiological measurements. These sensors have connectors which allow detachment from the instrument or cable from the instrument.